How can I delete the bevel object once a curve is created?
That will depend on the shape you want the bevel to be and whether you want to keep editing the curve. For odd shapes you will need to keep the bevel object as it defines the shape. You can hide the bevel object or move it to another layer to keep it out of the way. If you don't need to edit the tubing then you can setup your bevel and then convert it to a mesh with &#9095; AltC and delete the curve objects.

For simple bevel shapes you don't need to use a bevel object. A square tube can be created by adding a bevel depth and leaving the resolution at 0.



By increasing the resolution to 1 or 2 you get a hexagonal or octagonal shaped tubing, higher values give a rounded cylindrical tube.



If you want sharper edges you can add an edge split modifier.